chore(spanner): add capacity limit and CLOCK eviction to KeyRecipeCache

[olavloite]

• Bound query key recipes to prevent unbounded memory growth from dynamic ad-hoc queries.
• Implement scan-resistant CLOCK (Second-Chance) eviction using lock-free atomic reference tracking on shared read lookups.
• Remove redundant inner Arc wrapper in KeyRecipeCache to align with workspace synchronization standards.
• Add unit tests verifying capacity constraints, second-chance survival, and scan resistance against query floods.

[gemini-code-assist]

Code Review

This pull request introduces a bounded CLOCK (Second-Chance) eviction mechanism for SQL query recipes in the KeyRecipeCache to prevent unbounded memory growth. It replaces the outer Arc wrapping of RecipeStore with a direct RwLock, establishes a default query capacity of 2,000, and tracks referenced entries using lock-free atomic stores. Feedback on the changes suggests an improvement to the insert_query function: when overwriting an existing query recipe, the referenced bit should be set to true to prevent the freshly updated entry from being prematurely evicted. A refactored implementation using the Entry API was provided to handle this cleanly.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request implements a bounded CLOCK (Second-Chance) eviction cache for SQL query recipes in KeyRecipeCache to prevent unbounded memory growth. It introduces a capacity limit (defaulting to 2,000) and tracks query references using lock-free atomic flags. Feedback on the implementation highlights a potential "lost insert" bug under high cache load, where newly inserted entries initialized with referenced: false can be immediately evicted during the same insertion cycle. Initializing new entries with referenced: true is recommended to resolve this issue.

[codecov]

Codecov Report

❌ Patch coverage is 98.28326% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.39%. Comparing base (4084332) to head (fa58fdb).

Files with missing lines	Patch %	Lines
src/spanner/src/routing/key_recipe_cache.rs	98.28%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6507      +/-   ##
==========================================
- Coverage   96.39%   96.39%   -0.01%     
==========================================
  Files         301      301              
  Lines       84762    84980     +218     
==========================================
+ Hits        81710    81920     +210     
- Misses       3052     3060       +8     

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